It is known for the acoustics of a certain environment to be analysed using a measuring system comprising a multitude of spatially distributed loudspeakers, wherein a single microphone is positioned at a certain central location.
In an alternative setting, a measuring system for analysing the acoustics of an environment may comprise a multitude spatially distributed microphones, wherein at a central location, a single loudspeaker is positioned.
Due to the reciprocal character of sound transmission, a sound source can be replaced by a microphone and a microphone by a sound source, with a performed measurement still providing the same information. Therefore, performing acoustics measurements with a single-loudspeaker system as described above is in principle able to provide the same information as performing such measurements with a single-microphone system.
In both types of measuring systems, it is important for the central device to behave as much as possible in an omnidirectional way, so that sound or a certain level is received by a microphone or emitted by a loudspeaker across the whole three-dimensional space surrounding the device.
Whereas for a microphone, omnidirectionality is relatively easily achievable by making it smaller, for a loudspeaker, the solution is less simple. This is related to the fact that a loudspeaker needs to have a certain size to provide sound of a sufficient level. Furthermore, a loudspeaker needs to have an certain interior space of certain size to be able to provide sound of a sufficiently low frequency.
For obtaining omnidirectionality in a loudspeaker, a standard solution is to provide it with a multitude of sound generators, wherein each sound generator is configured to emit sound in a different direction. With the multiple sound generators being arranged in a single housing, such a loudspeaker is provided with sufficient size and internal volume to generate sound of a sufficiently high level and sufficiently low frequency, while at the same time sound of sufficient level can be emitted towards to the whole of the surroundings of the device.
An example of a loudspeaker currently used in acoustic analyses, that to a relatively large extent behaves in an omnidirectional way, comprises a housing of a substantially dodecahedral shape, wherein a hole is provided in each of the walls of the housing lining a face of the dodecahedral shape, wherein a multitude of sound generators is provided, each sound generator lining one of the holes provided in the walls of the housing.
It has been found the degree of omnidirectionality of the dodecahedral loudspeaker decreases with frequency. For frequencies higher than a certain limit, a difference of several dB in sound level has been found to exist between various positions at a same radial distance away from the device, which is too large a difference to provide sufficiently accurate measurements. The dodecahedral loudspeaker furthermore has a relatively complex structure, and is relatively expensive.
Certain types of loudspeakers are known to be provided with a sensor for measuring a property representative of the strength of a sound source created by that loudspeaker. Such a sensor may for instance be configured to measure an air volume displacement or an air pressure difference generated by a sound generator of the loudspeaker in exciting a sound wave.
Loudspeakers provided with a sensor as described that are currently known are either small in size, providing a high degree of omnidirectionality, but only in combination with a low output level, or they are large in size, providing a high output level, but only in combination with a low degree of omnidirectionality. Loudspeakers as described are not known to provide a relatively high degree of omnidirectionality in combination with a high output level, or to provide a degree of omnidirectionality that is at least suitable for sufficiently accurate measurements using relatively high frequencies.
WO 96/11558 describes a device for creating an omnidirectional sound source comprising a loudspeaker which in one direction radiates into a hollow coupler with an open inlet communicating with and being closed by said loudspeaker as well as with an open outlet for sound to radiate from, said coupler comprising walls which are of a sectional area decreasing in a direction away from the loudspeaker, wherein the loudspeaker in the opposite direction radiates into a cabinet, wherein the sectional area of the cabinet decreases in a direction away from the loudspeaker. With this configuration, the outlet for sound to radiate from is of a substantially smaller diameter than the loudspeaker.
The company Bruel & Kjaer sells a device for creating an omnidirectional sound source comprising a loudspeaker radiating through a conical coupler to a circular orifice, wherein an adapter has been added at the output to measure the output volume velocity.